1.3.2.4 Summary of hydrology and water resources

Changes in river discharge, as well as in droughts and heavy rains in some regions, indicate that hydrological conditions have become more intense. Significant trends in floods and in evaporation and evapotranspiration have not been detected globally. Some local trends in reduced groundwater and lake levels have been reported, but these are likely to be due to human activities rather than climate change. Climate-change signals related to increasing runoff and streamflow have been observed over the last century in many regions, particularly in basins fed by glaciers, permafrost and snow melt. Evidence includes increases in average runoff of Arctic rivers in Eurasia, which has been at least partly correlated with climate warming, and earlier spring snow melt and increase in winter base flow in North America and Eurasia due to enhanced seasonal snow melt associated with climate warming. There are also indications of intensified droughts in drier regions. Lake formation and their subsequent disappearance in permafrost have been reported in the Arctic. Freshwater lakes and rivers are experiencing increased water temperatures and changes in water chemistry. Surface and deep lake waters are warming, with advances and lengthening of periods of thermal stability in some cases associated with physical and chemical changes such as increases in salinity and suspended solids, and a decrease in nutrient content.

1.3.3 Coastal processes and zones

Many coastal regions are already experiencing the effects of relative (local) sea-level rise, from a combination of climate-induced sea-level rise, geological and anthropogenic-induced land subsidence, and other local factors. A major challenge, however, is to separate the different meteorological, oceanographic, geophysical and anthropogenic processes affecting the shoreline in order to identify and isolate the contribution of global warming. An unambiguous attribution of current sea-level rise as a primary driver of shoreline change is difficult to determine at present.

Global sea level has been rising at a rate of about 1.7 to 1.8 mm/yr over the last century, with an increased rate of about 3 mm/yr during the last decade (Church et al., 2004; Holgate and Woodworth, 2004; Church and White, 2006; Bindoff et al., 2007, Section 5.5).